Generally, bar code indicia which includes contrast changes, typically black bars to white bars and white bars to black bars, are scanned using reflective illumination techniques. For instance, U.S. Pat. No. 4,698,514 discloses bar code markings on webs read reflectively. Reflectively reading bar codes involves illuminating the bar code using a light source, typically a light emitting diode (LED) or laser diode, and collecting indirect scattered reflections back into a scanning system. The collected reflections are focused on a photoelectric detector of some kind. Most systems of this type are designed and fixed to work over a broad range of print contrast between the bars and spaces of the bar code indicia or symbols. Some systems do provide for the ability to adapt themselves to changes in contrast or reflectance. One technique may be the use of automatic gain control system (AGC) in the input circuitry of the sensor as described, for instance, in U.S. Pat. Nos. 3,949,233, 4,528,444, and 5,248,871. Further, U.S. Pat. No. 4,985,636 discloses the use of variable load resistors under microprocessor control again to change the gain in the input circuitry of the sensor. Moreover, U.S. Pat. No. 5,550,362 describes varying the LED current in order to calibrate a bar code scanner to a known signal and target during manufacturing.
Scanning a bar code in transmission mode, on the other hand, involves illuminating one side of a web, having the light pass through the web, and collected on the other side of the web into a photoelectric device of some kind. As a dark element in the bar code passes between the LED and sensor, the amount of light which transmits through the web is reduced. The bar code may be applied to the web using any number of printing methods (for example, wet ink processes, ink jet, thermal transfer, clear or semi-opaque label, etc.) as long as the dark elements applied to the web absorb a portion of the light passing between the LED and sensor. Several prior art references disclose, to one extent or another, point to point illumination (LED to sensor) being used to detect the presence or absence of some blocking media, to wit: U.S. Pat. No. 4,097,731 to Krause et al.; U.S. Pat. No. 4,097,732 to Krause et al; and U.S. Pat. No. 5,289,011 to Wong et al. Moreover, these references also show the use of gain control or LED current control in presence/absence applications in order to compensate for environmental changes (dust and dirt buildup, or changes in illumination intensity due to component aging).
Although some progress has been made in the art for effectively reading bar codes on web, there persists a need in the art for an apparatus and method for reading bar codes on a moving web having a range of optical densities which uses a controllable light transmissive mode that is reliable, easy to use and manufacture and that is adaptable to enormously high production speeds.